We have examined the elementary molecular processes responsible for proton transfer and HD exchange in thin ice films for the temperature range of 100-140 K. The ice films are made to have a structure of a bottom D(2)O layer and an upper H(2)O layer, with excess protons generated from HCl ionization trapped at the D(2)OH(2)O interface. The transport behavior of excess protons from the interfacial layer to the ice film surface and the progress of the HD exchange reaction in water molecules are examined with the techniques of low energy sputtering and Cs(+) reactive ion scattering. Three major processes are identified: the proton hopping relay, the hop-and-turn process, and molecular diffusion. The proton hopping relay can occur even at low temperatures (<120 K), and it transports a specific portion of embedded protons to the surface. The hop-and-turn mechanism, which involves the coupling of proton hopping and molecule reorientation, increases the proton transfer rate and causes the HD exchange of water molecules. The hop-and-turn mechanism is activated at temperatures above 125 K in the surface region. Diffusional mixing of H(2)O and D(2)O molecules additionally contributes to the HD exchange reaction at temperatures above 130 K. The hop-and-turn and molecular diffusion processes are activated at higher temperatures in the deeper region of ice films. The relative speeds of these processes are in the following order: hopping relay>hop and turn>molecule diffusion.

The effect of hydroxide ions on proton transfer and H/D isotopic exchange of water molecules was examined at the surface of amorphous ice films at temperatures of 92-140 K. Excess hydroxide ions were provided onto a D(2)O-ice film by the hydrolysis of Na atoms, and H(2)O was adsorbed onto the surface for a submonolayer coverage. The H/D isotopic exchange between H(2)O and D(2)O molecules on the ice film surface was monitored as a function of reaction time and temperature by using the techniques of reactive ion scattering and low-energy sputtering. The result was compared with that obtained on a hydroxide-free ice film. At a temperature of 92 K, proton transfer occurred from water mostly to adjacent hydroxide ions. The proton transfer distance and the H/D exchange reaction rate increased with increase in temperature above 105 K. The H/D exchange reaction propagated to several water molecules on the surface at 100-120 K. Kinetic measurement in this temperature range deduced the Arrhenius activation energy for the reaction, E(a) = 9.6+/-2.0 kJ mol(-1). The study shows that hydroxide ions promote the H/D exchange reaction on the ice surface compared with that on a hydroxide-free ice surface, but the promotion effect is moderate and the H/D exchange occurs on a substantial energy barrier. It is suggested that the stabilization of hydroxide ions at the ice surface produces an energy barrier for the proton transfer.